Bags-List Pallet

A semi-sorted list, where items hold an AccountId based on some Score. The AccountId (id for short) might be synonym to a voter or nominator in some context, and Score signifies the chance of each id being included in the final [SortedListProvider::iter].

It implements [frame_election_provider_support::SortedListProvider] to provide a semi-sorted list of accounts to another pallet. It needs some other pallet to give it some information about the weights of accounts via [frame_election_provider_support::ScoreProvider].

This pallet is not configurable at genesis. Whoever uses it should call appropriate functions of the SortedListProvider (e.g. on_insert, or unsafe_regenerate) at their genesis.

Goals

The data structure exposed by this pallet aims to be optimized for:

• insertions and removals.
• iteration over the top* N items by score, where the precise ordering of items doesn't particularly matter.

Details

• items are kept in bags, which are delineated by their range of score (See [Config::BagThresholds]).
• for iteration, bags are chained together from highest to lowest and elements within the bag are iterated from head to tail.
• items within a bag are iterated in order of insertion. Thus removing an item and re-inserting it will worsen its position in list iteration; this reduces incentives for some types of spam that involve consistently removing and inserting for better position. Further, ordering granularity is thus dictated by range between each bag threshold.
• if an item's score changes to a value no longer within the range of its current bag the item's position will need to be updated by an external actor with rebag (update), or removal and insertion.